Trapping and detrapping of electrical charges in polymers

The major role played by space charge behaviour in a dielectric material for insulation is now well established. Electrical charge trapping and detrapping are the relevant phenomena which have to be studied. The trapping ability of materials can be measured using for example the mirror method. This method allows one to determine the trapped charge (Qp) close to the surface of an insulating sample after irradiation by a controlled high energy electron beam. We carried out extensive work aiming at a better understanding of space charge behaviour linked to structural parameters of polymers. As a result, trapping properties as a function of crystallinity, molecular weight or morphology were investigated. At the same time, effects linked to chemical modification of polymer (i.e. oxidation) were studied. All those results have shown that trapping and detrapping properties are closely related to structural parameters of the polymer. In more recent studies we intended to modulate the charging capacity of polyethylene by adding to it a given amount of a second polymer. It has been shown that by a careful control of the process, of the second polymer rate and structural properties, it was possible to modulate the charging behaviour of polyethylene without enhancing the value of tg δ which remains of the order of 10^-4. This class of materials could be a new direction towards improved breakdown properties